September 12th, 2018, 11:24 AM  #1 
Member Joined: Jul 2018 From: Germany Posts: 32 Thanks: 1  what is: sin(a+b)/sin(ab)
is there any simpler way to write this, I already looked at all the trigonometric identities, I couldn't find anything about fractions of sums. I also try to find: cos(ab)+cos(a+b) how do I do this, is there anything I'm missing? 
September 12th, 2018, 02:01 PM  #2 
Global Moderator Joined: May 2007 Posts: 6,628 Thanks: 622 
You can always write x=a+b and y=ab., get sin(x)/sin(y) or cps(y)/cos(x). You can't do anything more, since the angles are completely arbitrary.

September 12th, 2018, 09:23 PM  #3 
Global Moderator Joined: Dec 2006 Posts: 19,887 Thanks: 1836 
sin(a + b) $\ \ $ tan(a) + tan(b)  =  sin(a  b) $\ \ $ tan(a)  tan(b) cos(A  B) + cos(A + B) ≡ 2cos(A)cos(B) 
September 13th, 2018, 05:14 AM  #4 
Member Joined: Jul 2018 From: Germany Posts: 32 Thanks: 1  
September 13th, 2018, 05:16 AM  #5 
Member Joined: Jul 2018 From: Germany Posts: 32 Thanks: 1  
September 13th, 2018, 02:39 PM  #6 
Global Moderator Joined: May 2007 Posts: 6,628 Thanks: 622  
September 14th, 2018, 03:08 PM  #7 
Member Joined: Jul 2018 From: Germany Posts: 32 Thanks: 1  